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McMahon started his youth career with Belvedere FC.[2] McMahon then began his career as a trainee with West Ham United, turning professional in August 2002. In need of first team experience, he joined League TwoTorquay United on loan in March 2004,[3][4] making his debut as a late substitute for Martin Gritton in the 2–2 home draw with Yeovil Town on 3 April 2004. However, that was to be his only game for the "Gulls" as he returned to West Ham two weeks early, seemingly at his own request.[5]

He was released by West Ham at the end of the 2003–04 season, joining Martin Foyle's League OnePort Vale on non-contract terms in September 2004. He struggled to establish himself at Vale Park and dropped down a division to join Leyton Orient on a free transfer two months later, after rejecting a further short-term deal at Port Vale.[6] Impressing on a non-contract basis he signed a six-month deal with Orient in January 2005.[7]

In June 2011, he signed for Boreham Wood as a player-coach on a two-year contract.[17] McMahon's stay at Boreham Wood lasted seven months, as in January 2012 he signed for fellow Conference South club Eastleigh for an undisclosed fee.[18] The "Spitfires" went on to post a mid-table finish in 2011–12. He was sold to league rivals Dover Athletic for an undisclosed fee in September 2012.[19] Eastleigh manager Richard Hill had rejected two bids from the "Whites", before Dover boss Nicky Forster put in a bid that Eastleigh director Stewart Donald felt "matched our valuation" and represented "good value".[20] He played in the 3–2 play-off final defeat to Salisbury City on 12 May 2013.[21]

In June 2013, he signed for Ebbsfleet United for a fee of £13,000.[22] He became a key part of the club's passing style, but credited the club's success to manager Steve Brown.[23][24] The "Fleet" finished fourth in 2013–14 and reached the play-off final, where they were beaten 1–0 by Dover Athletic;[25] this was McMahon's fourth play-off final defeat in six years. He did though play in the Kent Senior Cup final, which ended in a 4–0 victory over Dover.[26]

He re-signed with Boreham Wood in January 2015 on a contract lasting until summer 2017.[27]

McMahon took charge at Conference South club Ebbsfleet United on a caretaker basis for three games in November 2014, winning two games and drawing one game in the period following the departure of Steve Brown and the appointment of Jamie Day.[27] He was given a two-year management contract at the club in April 2015.[28] He led the club to a second-place finish in the 2015–16 season, however his record of play-off disappointment continued into his management career as Ebbfleet went on to lose to Maidstone United on penalties in the play-off final.[29] The "Fleet" again finished second in the 2016–17 season, finishing only two points behind champions Maidenhead United. However they made up for their previous season's defeat and justified their 96-point finish after coming from 1–0 down with ten men to record a 2–1 victory over Chelmsford City in the play-off final at Stonebridge Road.[30]